The present invention relates to an embedding material based on a polyurethane produced from castor oil and an aromatic diisocyanate, modified by a glycol, and also to the process for producing the embedding material and its particular utility as a support for membranes.
A large number of embedding materials are known, especially synthetic polymerized compositions, and these embedding materials are used for various structural purposes, e.g. as a means of enclosing, fastening, strengthening, reinforcing, stabilizing or protecting any number of distinct articles, structural components or the like. The particular article or member is usually at least partly enclosed by the embedding material which must be relatively rigid and strong and also firmly adhered to and in intimate contact with the embedded member.
For the specific purpose of embedding membranes, for example, the thin-walled sheets, tubular foils or hollow fibers used in dialysis equipment, a variety of natural and synthetic substances have been recommended. In the German published application (DE-AS) No. 1,236,476, it is disclosed that such membranes, formed as fine hollow fibers and collected into several partial bundles, can be cemented into holders with the help of an embedding material based upon epoxy resins. Besides these epoxy resins, other materials recommended in the literature for use as an embedding material include, for example, elastomeric or resilient materials such as rubber, cork, foam rubber, etc., or also other resins such as phenol/aldehyde resins, melamine/aldehyde resins or heat-hardenable synthetic elastomers, acrylic resins and the like.
The embedding material most closely related to that used in the present invention has been disclosed in the German published application (DE-AS) No. 2,340,661 and consists of a flowable preadduct or prepolymer produced by reaction of an aromatic diisocyanate with castor oil which is then cross-linked with further castor oil after embedding with the prepolymer has occurred. Similar compositions have been used for the potting or encapsulation of electronic components, for example, as described in Chapter 10 of "Polyurethanes" by Bernard A. Dombrow, Reinhold Publ. Corp., N.Y. (1957) pp. 157-162. Among these potting materials, there is recommended at page 158 a nonfoamed solid polyurethane obtained by reacting castor oil/tolylene diisocyanate/ethyl hexanediol-1,3 in a molar ratio of 1.66:3.50-4.00:1.00, using Sylon RD-602 (ditertiary butoxy diamine silane) as the curing agent. While found to be an excellent material for embedding electronic components, it was very difficult to obtain bubble-free products resistant to mechanical failure. A few other encapsulating materials using castor oil/diisocyanate mixtures are suggested in this Dombrow reference, the cross-linking or curing agent being a propylene oxide addition product of hexamethylene diamine with ethyl diethanol amine as a curing accelerator. Low-branched polyesters, e.g. as used to manufacture flexible foamed polyurethanes, have been suggested as a replacement for the castor oil in the embedding compositions.
In general, the polyurethane embedding materials obtained from castor oil/diisocyanate prepolymers require a long reaction time for hardening and the resulting cured product is too soft for many purposes. In order to obtain an embedding material of sufficient hardness, the polyurethane prepolymer has been catalytically hardened. The resulting embedding material then contains catalysts, however, making the insertion or emplacement of the object being embedded very damaging or even impossible. Also, such catalyst additions tend to cause a higher swelling of the known embedding materials and an undesirable discoloration.